Perpendicular recording has been developed in part to achieve higher recording density than is realized with longitudinal recording devices. A PMR write head typically has a main pole layer with a small surface area at an air bearing surface (ABS) and coils that conduct a current and generate a magnetic flux in the main pole that exits through a write pole tip and enters a magnetic media (disk) adjacent to the ABS. The flux may return through a shield structure to the back gap region which connects the main pole with the shield structure. There is typically one or more write shields on the write gap layer above the main pole and along the ABS and an upper section of the shield structure which may have an arched shape is formed over the coil layer and connects the one or more write shield sections along the ABS to the back gap region.
Perpendicular magnetic recording has become the mainstream technology for disk drive applications beyond 150 Gbit/in2. The demand for improved performance drives the need for a higher areal density which in turn calls for a continuous reduction in transducer size. A PMR head which combines the features of a single pole writer and a double layered media (magnetic disk) has a great advantage over LMR in providing higher write field, better read back signal, and potentially much higher areal density. Typically, today's magnetic head consists of a writer and a reader as separate elements that are formed adjacent to one another along an ABS. The read head may be based on a TMR element in which a tunnel barrier layer separates two ferromagnetic (FM) layers where a first FM layer has a fixed magnetization direction and the second FM layer has a magnetic moment that is free to rotate about a direction orthogonal to the direction of the magnetic moment in the reference “fixed” layer. The resistance across the barrier changes as the free layer moment is rotated. This signal is used to detect the small magnetic field from the recorded magnetization pattern on the media.
Reducing the magnetic spacing from read/write heads to the magnetic media during both writing and reading is the most important factor in achieving better performance in high density recording. The writer and reader are separated by several microns in a typical recording head and are made of several different materials each having a unique CTE. Therefore, the protrusion of the reader and writer are usually quite different due to the effect of varying operating temperatures, applying dynamic flying height (DFH) power to actuate the reader or writer, or from write current excitation. In addition, the point with minimum spacing to disk could be located away from either the reader or the writer, imposing further restrictions to achievable magnetic spacing during reading and writing. Improvements in PMR head design are needed to control the protrusion differences at the writer, the reader and the minimum point, and its variation. In particular, for the touch down and then back off mode of operation using DFH, if the writer protrusion is much more than the reader protrusion, then the minimum reader spacing is determined by the excess protrusion plus any initial protrusion. The ratio of reader protrusion rate/writer protrusion rate is called the gamma ratio. A lower gamma ratio means the writer protrusion rate is much higher than the reader protrusion rate, and could potentially put a greater limit to achievable reader spacing.
An important head design objective is to achieve a gamma ratio as close as possible to 1 which is ideal for tribology and magnetic performance since it keeps the gap between reader and writer at a constant value independent of the DFH power used for actuation. From a drive reliability point of view, the reader should not be at the minfly point which is the mechanically closest part of the head to the disk because the read sensor is more sensitive to mechanical impact. But the additional spacing margin for the reader needs to be kept to as small a number as reliability allows in order to have the best read back performance possible.
Typically, this “dynamic” control of spacing involves a thin layer of heater film that is embedded inside the magnetic recording head. The joule heating from the electrical current into the heater film is conducted away from the source to the entire slider body and the air bearing surface (ABS) elastically deforms so the read gap (RG) and write gap (WG) of the recording head become closer to the recording media (disk).
The recent advancement of the touchdown detection scheme when the recording head touches the disk makes it possible to control the spacing accurately to well within a nanometer. Particular interest is focused on the differential protrusion rate of RG, WG, and the minimum flying point (MIN). Improvement of the gamma parameter for RG (RG protrusion rate/MIN protrusion rate) as well as the gamma parameter for WG is critical to the overall performance in resolution, signal-to-noise ratio (SNR) and bit error rate (BER).
A common way to increase the RG actuation during DFH operation is to increase the lower read shield (S1) thickness. The increased volume of the S1 enables the RG to protrude more at the same power, thus improving the RG gamma parameter and dynamic performance (DP). The thickness of the upper read shield (S2A) is part of the contribution to reader-writer separation which is desired to be as small as possible in order to have high format efficiency in the drive. With the increased imbalance of the S1 and S2A thickness resulting from a thicker S1, certain drawbacks in magnetic characteristics associated with the read shield thickness ratio create undesirable transfer curves for the reader. One drawback is an increased hysteresis reject rate during quasi-static (QST) testing. In addition, a thicker S1 reduces the QST amplitude for a fixed field span testing. This indirectly impacts QST based noise testing such as PAT (proportional amplitude testing). Although this issue can be addressed in principle by new testing conditions, significant investment would be required for appropriate tester upgrades. Thus, an alternative to a thicker S1 layer is desirable in order to improve RG actuation without adversely compromising other read head characteristics.
A search of the prior art revealed the following references that relate to read gap modification.
U.S. Pat. No. 6,700,752 describes a thermally conducting non-magnetic layer that is inserted in a read gap and adjoins a surface of the S1 shield that faces the sensor. The inserted layer reduces thermal resistance between the read sensor and nearby shield thereby allowing more power to be dissipated without overheating.
U.S. Pat. No. 5,811,018 discloses an interleaved magnetic head to control read gap thickness. A protective layer adjacent to the S2 shield is used to reduce the cost of the interleaved head and allows improved control of the read gap thickness.
In related patent application Ser. No. 12/080,276, a dual heater scheme is disclosed and is used to optimize the gamma ratio by independently controlling read gap protrusion and write gap protrusion.